This invention relates to an austenitic stainless steel valve used at high temperatures (e.g., in the chemical industry) and which is easy to operate at the start of its use.
FIGS. 1 and 2 show Y-type globe valves, which are typical austenitic stainless steel valves used at high temperatures, typically; at 100.degree. C. or higher. These valves each have a valve body 1 having a passage 2 formed therethrough and provided with a seat 3. A stem 4 is axially movably mounted on the valve body 1. A gland plate 7 and a gland bolt 8 compress and retain a gland packing 6 sandwiched between the stem 4 and a cover flange 5. A yoke 9 is fixed to the valve body 1 so as to extend parallel to the stem 4. A yoke flange 10 is mounted around one end of the stem 4 and fixed to the yoke 9. Rotatably inserted in the yoke flange 10 is a sleeve 11 formed with female threads which in mesh with male threads 13 formed on the stem 4. A handle 14 is fixed to the sleeve 11 to turn it. A rotation stopper 15 fixed to the stem 4 prevents rotation of the stem 4 by abutting the yoke 9. The stem 4 is moved by engagement with the sleeve 11.
The valve of FIG. 1 has a separate disk 16 mounted to the tip of the stem 4. The valve of FIG. 2 has a disk 16 integrally formed at the tip of the stem 4. When the sleeve 11 is turned by turning the handle 14, the stem 4 is moved axially because it cannot rotate. When the stem 4 is advanced, the disk 16 at the tip of the stem is pressed against the seat 3, stopping the fluid flow through the passage 2.
Thus, in this type of valve, while the valve is closed with the stem 4 compressed between the seat 3 and the sleeve 11 of the yoke flange 10, a compressive stress acts on the valve parts.
The female threads 12 of the sleeve 11 and the male threads of the stem 4 have a smaller lead than the friction angle so that the stem 4 will not loosen under reaction force from the seat 3,(i.e., move while the valve is closed).
The valves of FIGS. 1 and 2 are manual valves. However a pneumatic means or an electric motor may be used to axially move the stem 4. Structurally, such "power" valves have practically nothing different from the valves shown.
If such a valve is intended to be used at high temperatures, its valve body 1 is made from an austenitic stainless steel, and a heating jacket 17 is provided around the valve body 1.
To use such a valve at high temperatures, the valve has to be first heated to a predetermined temperature.
The austenitic stainless steel, from which the valve body 1 is made, has a high thermal expansion coefficient. 6 When the valve is heated in a closed state, the valve body 1 and the stem 4 are heated to higher temperatures than the yoke 9. The stem 4 is thus thermally expanded to a greater degree than the valve body 1 and the yoke 9 combined. If the valve were closed when heated, the seat 3 might be destroyed, and/or the stem 4 might be bent due to excessive compressive stress acting on the stem 4.
Thus, in order to prevent excessive stress acting on the stem, the valve has to be kept slightly open while the valve is being heated.
The valve body 1 is made from an austenitic stainless steel to improve corrosion resistance. Thus, if the valve body 1 is made from an austenitic stainless steel, it is an ordinary practice to also make other parts that are brought into contact with fluid, such as the stem 4, from an austenitic stainless steel. The yoke 9, which is kept out of contact with fluid, is usually made from inexpensive carbon steel.
For a valve of this type, the compressive stress that acts on the stem 4 when the valve is heated to 300.degree. C. with the valve closed.
In our experiment, when the valve body 1 was heated to 300.degree. C., the temperature of the stem 4 and the yoke 9 at their exposed portions were 80.degree. C. and 50.degree. C., respectively.
If this valve is closed at 20.degree. C. and heated to 300.degree. C., then the valve body temperature changes 280.degree. C. (=300-20). The stem temperature, which is herein assumed to be the average of temperatures at its hottest and coldest portions, changes 170.degree. C.=(300+80)/2-20. The yoke temperature changes 30.degree. C. (=50-20).
Supposing that the length from the seat 3 of the valve body 1 to the yoke 9 mounting portion is L, the length of the yoke 9 was 1.5 L and the length from the seat 3 of the stem 4 to the driving screw portion was 2.5 L.
The thermal expansion amounts at the respective parts are calculated below:
Stem 4: EQU 170.times.1.7.times.10.sup.-5 .times.2.5L=7.225.times.10.sup.-3 L
Valve body 1 and yoke 9 combined: EQU 280.times.1.7.times.10.sup.-5 .times.L+30.times.1.1.times.10.sup.-5 .times.1.5L =5.255.times.10.sup.-3 .times.L
wherein 1.7.times.10.sup.-5 /.degree. C. and 1.1.times.10.sup.-5 /.degree. C. are thermal expansion coefficients of an austenitic stainless steel and a carbon steel at their respective temperatures. From the above equations, it is apparent that the stem 4 will expand by 1.970.times.10.sup.-3 .times.L longer than the combined elongation of the valve body 1 and yoke 9. Since the modulus of longitudinal elasticity of an austenitic stainless steel is about 21000 kg/mm.sup.2, the compressive stress that acts on the stem 4 at this time will be: EQU .sigma.=21,000.times.1.970.times.10.sup.-3 =41.4 kg/mm.sup.2
At 200-300.degree. C., the allowable stress for an austenitic stainless steel is about 10 kg/mm.sup.2 at the most. Thus, it is apparent that the stem cannot withstand such a large stress.
Since the seat 3 and the stem head 16 are tapered, the stem head will cut into the seat if the difference in thermal expansions of the respective parts is large as above. Thus, this results in the seat 3 being destroyed, and/or the stem 4 being bent under excessive compressive stress. In either case, the valve fails. Thus, when such a valve is heated, it is essential to keep the valve slightly open.
An object of the present invention is to provide an austenitic stainless steel valve which is easy to operate at the beginning of use.